1. Field of the Invention
The present invention relates to TDMA (Time Division Multiple Access) cordless telephone systems, and in particular, to an intercommunication method between portable units.
2. Description of the Related Art
In general, a cordless telephone system is provided with a cordless base unit (referred to as base unit) and a plurality of cordless portable units (referred to as portable units). Recently, many users have registered two or more portable units with a base unit in the cordless telephone system. This cordless telephone system offers the function of an intercommunication between portable units. That is, voice communication between a calling portable unit and a remotely located called portable unit. This intercommunication is available via the base unit and will be described below with reference to FIG. 1.
FIG. 1 illustrates a base unit and a plurality of portable units (four portable units here) registered with the base unit in a cordless telephone system. Two portable units CP.sub.-- 1 and CP.sub.-- 2 are positioned within a propagation area (or coverage area) 10 of a radio signal transmitted from the base unit, and two portable units CP.sub.-- 3 and CP.sub.-- 4 are located beyond the propagation area. The base unit is connected to or disconnected from a main line (i.e., tip and ring terminals) depending on an off- or on-hook state. Upon connection to the main line, the base unit performs communication through a telephone wire. Upon reception of a call request from a portable unit, the base unit connects the portable unit to the main line for voice communication, and wirelessly transfers a signal received through the wire to the portable unit. Therefore, a portable unit intercommunicates with another portable unit or communicates with an external user on the public telephone network by way of radio communication with the base unit.
An example of the intercommunication in a TDMA cordless telephone system is as follows. When the first portable unit CP.sub.-- 1 within the propagation area is to call the second portable unit CP.sub.-- 2 also within the propagation area, the first portable unit CP.sub.-- 1 designates time slots to communicate with the base unit and sends a signal to the base unit in the time slots requesting a call to the second portable unit CP.sub.-- 2. Then, the base unit designates time slots unoccupied by the first portable unit CP.sub.-- 1 and calls the second portable unit CP.sub.-- 2 in the newly designated time slots. The second portable unit CP.sub.-- 2 receives a ring signal and can communicate with the base unit when a user has the second portable unit CP.sub.-- 2 off hook. The base unit transmits data received from the second portable unit CP.sub.-- 2 to the first portable unit CP.sub.-- 1 in the time slots designated for the first portable unit CP.sub.-- 1. Thus, intercommunication between the portable units can be conducted.
However, the conventional intercommunication method has a shortcoming in that portable units beyond the propagation area of the base unit cannot intercommunicate with each other in a TDMA cordless telephone system because there is no base unit for transferring a call between them.